This invention relates generally to gas-fired condensing furnaces, and more specifically to a corrosion resistant secondary or condensing heat exchanger which is both inexpensive and relatively easy to manufacture.
Current heat exchangers which are used in residential furnaces are expensive, highly tooled clamshell designs, and are not easily adapted to other furnaces. Flexible designs, such as those using tubing, require the use of stainless steels for corrosion protection and are also relatively expensive to manufacture. Attempts have been made to fabricate low cost corrosion-proof tubes by using polymer coatings or dipping techniques. These methods have been unsuccessful to date due to the difficulty in obtaining a "leak free" polymer coverage.
There is also a need for improvements to the headering and manifolding of the secondary heat exchanger. If "roll lock", or expansion joints could be made between the secondary heat exchanger and vestibule/primary heat exchanger a difficult gasketing and sealing process would be eliminated. In addition, existing heat exchanger design does not facilitate the use of fins (for increasing the air side heat transfer surface area). A tubing design which could provide the advantages described above, would provide a solution to some of the existing disadvantages associated with current clamshell designs.